Process oven fans are sized using the volume of the oven structure and a desired temperature gradient in the work area. The number of air changes per minute will result in the required air flow circulation for the fans. The burner is selected to heat this amount of air and the oven structure, plus the work to be baked or heat treated. Designers must decide if they wish to have single or multiple recirculation fans. The batch type oven usually uses smaller single fans and large, continuous-operation ovens usually use multiple fans. If the workspace for the oven includes considerable headroom, mounting the fans on top of the oven in a separate heater house usually results in saving floor space. In this larger oven case, a long shaft air kit is a clean, neat fan installation. If the fans must be mounted in the oven wall, the plug fan design is desirable. The plug fans are capable of higher efficiencies and high pressures. Multiple fans allow for counter rotation of air currents which some designers feel gives a better temperature gradient.